The Stirling dish systems are electricity generation units that use solar radiation as a source of energy. The capacity of a single unit is between 3 and 50 kWe.
The Stirling dish systems transform with high efficiency the concentrated solar radiation into electrical energy. The essential components of the system are:                Parabolic solar concentrator.        Solar heat exchanger (solar receiver).        Stirling Engine with electric generator.        Tracking system.        
The mode of operation of a Stirling dish system is the following: the concentrator reflects the solar radiation to the receiver which is located at the focal point of the concentrator. The solar radiation is absorbed in the receiver and this heats the gas (helium or hydrogen) of the Stirling engine at temperatures that are around 650° C. This heat is converted into mechanical energy in the Stirling engine. An electric generator converts this mechanical energy into electricity. So that the reflected radiation strikes the focal point during the entire day, a solar tracking system continually moves the concentrator to follow the path of the sun.
The technology of the solar receivers is developed depending on the type of process in which it will be used, i.e., the type of plant and the cycle used. The invention filed relates to the solar receiver plant with disc and the cycle is Stirling. Of course, it is interesting to know the background and previous developments used in solar applications. The technologies used for solar plants in tower receivers represent a reference application.
In particular, two types of receiver systems are used for the Stirling parabolic dish:                External receiver systems.        Cavity receiver systems.        
The external receivers have absorption surfaces in direct view with the concentrators and depend on the direct absorption of the radiation. The cavity receivers have, in turn, an opening through which the concentrated radiation passes to reach the surface of the receiver. The cavity ensures that most of the radiation that enters is absorbed by the internal surface of the receiver.
The receivers most widely used for the Stirling dish systems are the cavity receivers. The receiver is located behind the opening to reduce the amount of lost heat and to decrease the intensity of the flow concentrated on its surface. The concentrated radiation that enters through the opening of the receiver is spread inside the cavity. Most of the energy is absorbed directly by the receiver, and virtually all the remaining energy is reflected or irradiated again within the cavity to be absorbed later.
In a cavity receiver, two methods for transferring the solar radiation absorbed to the Stirling engine working fluid have been identified.
The first method consists of using a receiver of directly illuminated tubes where small tubes, through which the work fluid of the engine circulates, are placed directly in the region where the concentrated solar flow strikes. The tubes form the surface of the receiver. In this way the working gas is heated as it passes through the interior of the tubes heated by the solar radiation.
The second method, the reflux method, uses a liquid metal as intermediate heat transfer fluid. The liquid metal is vaporized on the back surface of the receiver and is condensed in the tubes through which the working fluid of the engine circulates. That is, it absorbs the heat from the material that forms the receiver (which is hot by the exposure to solar radiation) and then transfers it to the tubes through which the working gas of the engine circulates. This second type of receiver is called reflux receiver because the steam is condensed and returns to be evaporated again.
An important factor in the design of the receiver is the exposure to severe conditions of operation together with cyclical conditions. The high temperature is the most important factor which, together with cycle operation, gives rise to the thermal fatigue of the components. Thermal fatigue is caused by the temperature cycles, from room temperature to operation temperature, both in the starts and stops and during the moments of cloudiness. This type of cycle can cause premature failure of the receiver. Within the receiver-cavity system, the receiver component is particularly sensitive. The design of receiver tubes, incorporating thin walls and operating at even temperatures during the transient, usually has fewer problems with thermal fatigue. The long term creep of the material of the receiver and the oxidation are important considerations for choosing the materials. Chrome and nickel super alloys (Inconel®), stainless steels, titanium and nickel alloys, nickel and cobalt alloys, etc are commonly used.
There are numerous documents which develop different receivers or aspects of the same in the state of the art. Some of them are pointed out in the following:                DE4433203 1996 HTC Solar Solar heater head for generation of electric current from solar energy. Absorber with highly conductive material and blackened (oxidized) to homogenize the heat concentration. Preferably copper material, which is welded to the tubes of the heat exchanger with high performance stainless steel (as a sleeve of the tube).        US2002059798 2002 Midwest Research Institute Dish/stirling hybrid-receiver. Receiver hybrid system with sodium “heat pipe”. Structure of the element made of nickel powder. It describes integration with burner system (hybrid).        U.S. Pat. No. 6,735,946 2004 Boeing Direct illumination free piston stirling engine solar cavity. Directly illuminated piston. Without tube exchanger. Arrangement of small pistons concentrically to the solar beam. The receiver is a metallic element made of highly conductive material such as copper, nickel or graphite. An alternative to the design is to use “heat pipes”.        U.S. Pat. No. 6,739,136 2003 Boulder, Colo. Arvada, Colo. Combustion system for hybrid solar fossil fuel receiver. Combustion system for hybrid solar fossil fuel receiver comprising a premixer that combines air and fuel for forming the mixture to be burned. There is a heat exchanger associated to and in contact with the combustion chamber. This heat exchanger provides the heat for the hybrid receiver when the heat from the sun cannot be used as energy source.        U.S. Pat. No. 6,818,818 2004 Plano, Tex. Concentrating solar energy receiver. It describes a system consisting of a high reflectivity parabolic concentrator for reflecting the rays of the sun on its concave side and a conversion module that receives the concentrated solar radiation. In this conversion module, there are two different receivers; a photovoltaic receiver and a reception surface attached to a heat engine to produce electricity.        EP0996821 2000 STM Corporation Heat engine heater assembly. An equipment designed to use both solar radiation and the heat produced by the combustion of natural gas as a power source for a heat engine is described. A housing forming the receiver allows the input of solar radiation until reaching the absorber. Series of tubes external and internal to the receiver chamber absorb the solar radiation and transmit heat to the fluid flowing inside of it. A burner inside the chamber produces combustion gases that also heat up these tubes.        U.S. Pat. No. 4,665,700 1987 STM United Stirling AB Hot gas engine heater head. The object of the invention is to provide a heating head in which the regenerators are connected to the cylinders by tubes that surround these cylinders.        U.S. Pat. No. 4,602,614 1986 United Stirling, Inc. Hybrid solar/combustion powered receiver. It is an improved receiver which includes a heat exchanger inside the cavity with the tubes tangentially spaced. There are multiple burners to provide a path of the combustion gases and a window to seal the opening and so there are no gas leaks out of the receiver.        U.S. Pat. No. 6,668,555 2003 Boeing Company Solar receiver-based power generation system. This invention provides a design of improved solar receiver that reduces the cost of such mechanisms. The solar receiver includes a heat pipe that has a fluid inside of it. The heat pipe has two condenser parts arranged in two ends. Moreover, it includes an evaporator between both ends. An air collector is attached to one of the ends. This collector has an input and an output of air. A liquid collector is attached to the other end, with its respective input and output.        U.S. Pat. No. 4,911,144 1990 Stirling Thermal Engine, Inc Spherical solar energy collector. Invention relating to a collector for solar energy and in particular to one comprising an evaporator of a heat pipe-type heat transfer system        U.S. Pat. No. 4,475,538 1984 United Stirling AB Window for solar receiver for a solar-powered hot gas engine. Solar receiver which includes a window for the entry of the solar radiation as an improvement.        CA2490207A1 2004 Shecs Labs—Solar Hydrogen Solar energy collector. The invention is a receiver with an internal cavity that reflects radiation. The receiver is contained in an inert or reducing atmosphere to maintain the properties of the reflective surfaces of the cavity. The heat absorption occurs in tubes arranged symmetrically with respect to the main axis of the receiver. In addition there is a quartz window in the input of the device to reduce losses by convection.        DE19527272: Solarer Erhitzer für Stirling-Motoren. Solar heater (1) for Stirling engine with a field of absorption (2) of parallel tubes (8), which are connected to two collectors (4, 5) through which the working gas circulates. The tubes (8) are identical to each other and have loop-shape geometry.        
In view of the existing state of the art, the present invention aims to provide a solar receiver that, overcoming the deficiencies found in the previous designs:                increases the resistance to thermal fatigue,        minimizes the shadows between tubes,        has directly illuminated tubes, to simplify the system avoiding the inclusion of an intermediate heat transfer fluid and an additional heat exchanger as well as to make the optical design of the concentrator more flexible and that the engine can operate in other positions different than with the sun behind it,        does not leave gaps between the tubes when deforming by expansions, escaping the concentrated solar radiation through said gaps,        is easily weldable,        reduces head losses.        
Thus, the new design allows to enhance the efficiency of the disc and to reduce the manufacturing, operation and maintenance costs. It also offers the possibility of:                easily refrigerating in the case of overheating (fan),        making the receiver independent from the cavity and the housing of the insulation that could cover it, to make it more versatile and facilitate the maintenance,        having the possibility of integrating a possible hybridization gas burner,        simplifying the manufacturing processes and facilitating the construction.        